Project Summary To maintain mobility as we age we must navigate a complex environment that challenges us with a range of disturbances. Unfortunately, older adults are often unsuccessful in resisting these disturbances, and fall. Falls are the greatest source of injury, and for older adults, the greatest source of injury-related death. Approximately a quarter of falls happen following slips. Recent research on slips has shown that use of perturbation training to improve specific motor skills to resist slips can vastly reduce falls. However, these interventions have only addressed slips occurring at heel-strike, and not the diverse range of slipping disturbances presented by a complex environment. This project will focus on slips that occur across the gait cycle, and the reactive stabilization movements that follow. Slips at different phases of the gait cycle have unique biomechanical contexts, and successful reactive stabilization movements are likely to be highly specific to that context. Yet nothing is known about the specificity of the repertoire of reactive stabilization movements to resist different slip conditions. Furthermore, variability in the repertoire of reactive stabilization movements is likely to affect the success/failure rate of resisting these disturbances. Thus, the objective of the proposed project is to determine the roles of variability and specificity in reactive stabilization movements to resist falling in diverse slipping conditions. A novel wearable apparatus for slip perturbations will deliver lifelike, unexpected slips in early, middle and late stance in younger and older adults. Biomechanical analysis based on three- dimensional motion capture data of the reactive stabilization movements will generate novel results on the specificity and variability of protective stepping and arm swinging responses. The central hypothesis is that increases in specificity and decreases in variability of reactive stabilization movements will reduce fall rates. The first specific aim is to quantify variability and specificity of reactive responses to diverse slip perturbations and determine their relationships to fall rates. The second specific aim is to determine how repeated exposure to diverse slip perturbations affects reactive responses, and whether it relates to gait variability. The long-term objective of this research is to support future studies into the repertoire of reactive stabilization movements across the full range of disturbances faced in navigating a complex environment, and to inform future interventions that target a comprehensive resistance to falls as we age.